Heterogeneous photoconductor composition formed by two-stage dilution technique

ABSTRACT

HETEROGENEOUS PHOTOCONDUCTIVE COMPOSITIONS CONTAINING DYE AND POLYMER ARE PREPARED BY A TWO-STAGE DILUTION TECHNIQUE. THE DYE USED IS DISSOLVED IN A SOLVENT AT A DYE/SOLVENT RATION SUBSTANTIALLY EQUAL TO THE SOLUBILITY LIMIT OF THE DYE. POLYMERIC BINDER AND PHOTOCONDUCTOR ARE ADDED AND THE ADDITIONAL SOLVENT IS ADDED TO SUBSTANTIALLY REDUCE THE DYE/SOLVENT RATIO WELL BELOW THE SOLUBILITY LIMIT. UPON COATING THE RESULTANT FORMULATION, A HIGHSPEED HETEROGENEOUS COMPOSITION RESULTS.

United States Patent 3,679,408 HETEROGENEOUS PHOTOCONDUCTOR COM- POSITION FORMED B Y TWO-STAGE DILU- TION TECHNIQUE Frederick J. Kryman, Hornell, and William J. Staudenmayer, Pittsford, N.Y., asslgnors to Eastman Kodak Company, Rochester, NY. No Drawing. Filed Nov. 13, 1970, Ser. No. 89,448 Int. Cl. G03g 5/00; H011 13/00 U.S. Cl. 961.6 Claims ABSTRACT OF THE DISCLOSURE Heterogeneous photoconductive compositions containingdye and polymer are prepared by a two-stage dilution technique. The dye used is dissolved in a solvent at a dye/ solvent ratio substantially equal to the solubility limit of the dye. Polymeric binder and photoconductor are added and the additional solvent is added to substantially reduce the dye/solvent ratio well below the solubility limit. Upon coating the resultant formulation, a highspeed heterogeneous composition results.

subsequent operations, now well known in'the art, can

then be employed to produce a permanent record of the image.

One type of photoconductive insulating structure or element particularly useful in electrophotography utilizes a composition containing a photoconductive insulating material dispersed in a resinous material. A unitary electrophotographic element is generally produced in a multilayer type of structure by coating a layer of the photoconductive composition onto a film support previously overcoated with a layer of conducting material or the photoconductive composition may be coated directly onto a conducting support of metal or other suitable conducting material. Such photoconductive compositions have shown improved speed and/or spectral response, as well as other desired electrophotographic characteristics when one or more photosensitizing materials or addenda are incorporated into the photoconductive composition. Typical addenda of this latter type are disclosed in U.S. Pat. Nos. 3,250,615, issued May 10, 1966, by VanAllan; 3,141,770, issued July 21, 1964, by Davis et al.; and 2,987,395, issued June 6, 1961, by Jarvis. Generally, photosensitizing addenda to photoconductive compositions are incorporated to effect a change in the sensitivity or speed of a particular photoconductor system and/or a change in its spectral response characteristics. Such ic response of the photoconductor should be capable of reproducing the wide range of colors which are typically 3,679,408 Patented July 25, 1972 encountered in such use. If the response of the photoconductor falls short of these design criteria, it is highly desirable if the spectral response of the composition can be altered by the addition of photosensitizing addenda to the composition. Likewise, various applications specifically require other characteristics such as the ability of the element to accept a high surface potential, and exhibit a low dark decay of electrical charge. It is also desirable forthe photoconductive element to exhibit high speed as measured in an electrical speed or characteristic curve, a low residual potential after exposure and resistance to fatigue. Sensitization of many photoconductive compositions by the addition of certain dyes selected from the large number of dyes presently known has hitherto been widelyused to provide for the desired flexibility in the design of photoconductive elements in particular photoconductor-containing systems. Conventional dye addenda to photoconductor compositions have generally shown only a limited capability for over-all improvement in the totality of electrophotographic properties which cooperate to produce a useful electrophotographic element or structure. The art is still searching for improvements in shoulder and toe speeds, improved solid area reproduction characteristics, rapid recovery and useful electrophotographic speed from either positive or negative electrostatic charging.

A high speed heterogeneous or aggregate photoconductive system was developed by William A. Light which overcomes many of the problems of the prior art. This aggregate composition is the subject matter of copending application Ser. No. 804,266, filed Mar. 4, 1969, now U.S. Pat. No. 3,615,414, and entitled Novel Photoconductive Compositions and Elements. The addenda disclosed-therein are responsible for the exhibition of desirable electrophotographic properties in photoconductive elements prepared therewith. However, in accordance with the procedures described therein, the preparation of electrophotographic elements uses a solvent treatment step subsequent to the coating step. In an effort to avoid this secondary treatment step, a novel method of preparation of photoconductive compositions of the type described by Light is disclosed in copending Eugene P. Gramza application Ser. No. 821,513, now U.S. Pat. No. 3,615,415, filed May 2, 1969, and entitled Method for the Preparation of Photoconductive Compositions. This latter method involves the high speed shearing of the photoconductive composition prior to coating and thus eliminates subsequent solvent treatment. However, it is often desirable to have photoconductive compositions of even higher speeds than those obtainable with the above compositions. Thus, copending Edward I. Seus application Ser. No.'764,302, filed Oct. 1, 1968, now U.S. Pat. No. 3,591,374, and entitled High Speed Electrophotographic Elements and Method for Preparation Thereof discloses a technique for substantially increasing the speed of the above compositions. This technique involves forming electrophotographic layers by the above techniques and then overcoating such layers with a solution of suitable dye. This latter procedure uses a secondary coating step.

An additional problem encountered in forming such heterogeneous photoconductive compositions is that many of the useful dyes have several crystalline structures. Depending upon which crystalline structure is present, the aggregate formation can be relatively easy or quite difficult. This latter problem has been overcome by the procedure described in copending Eugene P. Gramza et al. application Ser. No. 816,831, now U.S. Pat. No. 3,615,396, filed Apr. 14, 1969, and entitled Method for the Preparation of Photoconductive Compositions. The technique described therein is the so-called dye-first method and involves the complete dissolution of the dye prior to addition of any other ingredients. This method is useful; however, it produces a fairly large sized aggregate, which for some applications is undesirable. Accordingly, there is a need for a method of producing heterogeneous photoconductive compositions which have a relatively small aggregate size.

It is, therefore, an object of this invention to provide the art of electrophotography with a novel method for preparing photoconductive compositions.

It is an additional object to provide a novel method for forming heterogeneous or aggregate photoconductive compositions containing dye and polymer, which method is independent of the crystalline structure of the dye and which results in smaller sized aggregates.

It is another object to provide a novel process for forming high speed electrophotographic elements.

These and other objects and advantages of the invention will become apparent from the following descrip tion of the invention.

It has been discovered that, when the heterogeneous or aggregate photoconductive compositions of William A. Light are prepared by adding the ingredients in a certain prescribed manner, formation of the heterogeneous composition is obtained without the necessity of any secondary treatment or additional ovencoating steps. Furthermore, when the present method is used, the formation of such photoconductive compositions is found to be independent of the crystalline structure of the dye or dyes used and it results in the formation of smaller, more uniform aggregates. In particular, when the coating dope is prepared by a two-stage dissolution technique, improved control of aggregate size and uniformity is obtained.

The method of this invention is used to form heterogeneous multiphase photoconductive compositions comprised of an organic sensitizing dye and an electrically insulating, film-forming polymeric material. The present method is relatively uncomplicated and provides results which are readily reproducible and which are relatively independent of the crystalline structure of the particular dye or dyes used. One of the essential features of the instant invention is the dissolution of the sensitizing dye in a suitable solvent at a dye-to-solvent ratio substantially equal to the solubility limit of the dye. This is done prior to the addition of any other add'enda. After dissolving the dye, the polymeric material is subsequently added with suitable stirring to dissolve the polymer. Additional solvent is then added to substantially reduce the dye-to-solvent ratio well below the solubility limit of the dye. "Of course, depending upon the final concentration of polymer and photoconductor desired, these may also be added. The combined solution is then coated on a suitable support which results in the formation of a separately identifiable multiphase composition, the heterogeneous nature of which is generally apparent when viewed under 2500 magnification, although such compositions may appear to be substantially optically clear to the naked eye in the absence of magnification. Suitably, the dye-containing aggregate in the discontinuous phase is submicron in size and is predominantly in the size range of about 0.01 to about 0.75 micron. However, it should be noted that when the heterogeneous compositions prepared by this invention are used to sensitize a particulate photoconductor, such as zinc oxide, another discontinuous phase will be present which may not fall within this size range.

In general, the heterogeneous compositions formed by the present method are multiphase organic solids. The polymer vehicle comprises an amorphous matrix or continuous phase which contains a discrete discontinuous phase as distinguished from a solution. The discontinuous phase is the aggregate species which is a co-crystalline complex comprised of dye and polymer. The term coci'ystalline complex as used herein has reference to a crystalline compound which contains dye and polymer molecules co-crystallized in a single crystalline structure 4 to form a regular array of the molecules in a three di mensional pattern.

When the present compositions are used in conjunction with an organic photoconductor, the resultant photoconductive composition generally contains only two phases as the photoconductor usually forms a solid solution with the continuous polymer phase. On the other hand, when the present multiphase compositions are used in conjunction with a particulate photoconductor, three phases may be present. In such a case, there would be a continuous polymer phase, a discontinuous phase containing the aggregate as discussed above and another discontinuous phase comprised of the particulate photoconductor. Of course, the present multiphase compositions may also contain additional discontinuous phases of trapped impurities, etc. Another feature characteristic of the heterogeneous compositions formed in accordance with this invention is that the wavelength of the radiation absorption maximum characteristic of such compositions is substantially shifted from the wavelength of the radiation absorption maximum of a substantially homogeneous dye-polymer solid solution formed of similar constituents. The new absorption maximum characteristic of the aggregates formed by this method is not necessarily an overall maximum for this system as this will depend upon the relative amount of dye in the aggregate. Such an absorption maximum shift in the formation of multiphase heterogeneous systems for the present invention is generally of the magnitude of at least about 10 nm. It mixtures of dyes are used, one dye may cause an absorption maximum shift to a long wavelength and another dye cause an absorption maximum to a shorter wavelength. In such cases, a formation of the heterogeneous compositions can more easily be identified by viewing under magnification.

Sensitizing dyes and electrically insulating polymeric materials are used in forming these heterogeneous compositions. Typically, pyrylium dyes, including pyrylium, thiapyrylium and selenapyrylium dye salts are useful in wherein R R R R and R can each represent (a) a hydrogen atom; (b) an alkyl group typically having from 1 to 15 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, amyl, isoamyl, hexyl, octyl, nonyl, dodecyl, etc., (c) alkoxy groups like methoxy, ethoxy, propoxy, butoxy, amyloxy, hexoxy, octoxy, and the like; and (d) aryl groups including substituted aryl groups such as phenyl, 4-diphenyl, alkylphenyls as 4-ethylphenyl, 4-propylphenyl, and the like, alkoxyphenyls as 4 ethoxyphenyl, 4 methoxyphenyl, 4 amyloxyphenyl, 2-hexoxyphenyl, Z-methoxyphenyl, 3,4-dimethoxyphenyl, and the like, B-hydroxyalkoxyphenyls as 2-hydroxyethoxyphenyl, 3-hydroxyethoxyphenyl, and the like, 4-hydroxyphenyl, halophenyls as 2,4-dichlorophenyl, 3,4- dibromophenyl, 4-chlorophenyl, -3,4-dichlorophenyl, and the like, azidophenyl, nitrophenyl, aminophenyls as 4-diethylaminophenyl, 4-dimethylaminophenyl and the like, naphthyl; and vinyl substituted aryl groups such as styryl, methoxystyryl, diethoxystyryl, dimethylaminostyryl, 1- butyl-4-p-dimethylaminophenyl-l,3-butadienyl, 13 ethyl-4- dimethylaminostyryl, and the like; and where X is a sulfur, oxygen or selenium atom, and Z" is an anionic function, including such anions as perchlorate, fluoroborate, iodide, chloride, bromide, sulfate, periodate, p-toluenesuL fonate, and the like. In addition, the pair R and R as well as the pair R and R can together be the necessary Typical useful polymers contain the following moiety in the recurring unit:

wherein:

R and R when taken separately, can each be a hydrogen atom, an alkyl radical having from one to about carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, tertiary butyl, pentyl, 'hexyl, heptyl, octyl, nonyl, decyl and the like, including substituted alkyl radicals such as trifiuoromethyl, etc., and an aryl radical such as phenyl and naphthyl, including substituted aryl radicals having such substituents as a halogen, alkyl radicals of from 1 to about 5 carbon atoms, etc.; and R and R when taken together, can represent the carbon atoms necessary to form a cyclic hydrocarbon radical including cycloalkanes such as cyclohexyl and polycyclo- :alkanes such as norbornyl the total number of carbon atoms in R and R being up to 19;

R and R, can each be hydrogen, an alkyl radical of from 1 to about 5 carbon atoms, e.g., methyl, ethyl, isopropyl, butyl, amyl, etc., or a halogen atom such as chloro, bromo, iodo, etc.; and

R is a divalent radical selected from the following:

wherein: each R is a phenylene radical including halo substituted phenylene radicals and alkyl substituted phenylene radicals and alkyl substituted phenylene radicals; and R and R are as described above. Such compositions are disclosed, for example, in U.S. Pat. Nos. 3,028,365 by Schnell et al., issued Apr. 3, 1962 and 3,317,466 by Caldwell et al., issued May 2, 1967. Preferably, polycarbonates containing an alkylidene diarylene moiety in the recurring unit such as those prepared with Bisphenol A and including polymeric products of ester exchange between diphenylcarbonate and 2,2-bis(4-hydroxypheuyl)propane are useful in the practice of this invention. Such compositions are disclosed in the following U.S. Patents: 2,999,750 by Miller et al., issued Sept. 12, 1961; 3,038,874 by Laakso et al., issued June 12, 1962; 3,038,879 by Laakso et al., issued June 12, 1962; 3,038,880 by Laakso et al., issued June 12, 1962; 3,106,544 by Laakso et al., issued Oct. 8, 1963; 3,106,545 by Laakso et al., issued Oct. 8, 1963; 3,106,546 by Laakso et al., issued Oct. 8, 1963; and published Australian patent specification No. 19,575/56. A wide range of film-forming polycarbonate resins are useful, with completely satisfactory results being obtained when using commercial polymeric materials which are characterized by an inherent viscosity of about 0.5 to 0.6. In addition, a high molecular weight material such as a high molecular weight Bisphenol A polycarbonate can be very useful. Preferably, such high molecular weight materials have an inherent viscosity of greater than about 1 as measured in 1,2-dichloroethane at a concentration of 75 0.25 g./ ml. and a temperature of about 25 C. The use of high molecular weight polycarbonate, for example, facilitates the formation of aggregate compositions having increased speeds.

The following polymers are included among the materials useful in the practice of this invention:

TABLE 2 Number Polymeric material 1 Poly(4,4-1sopropylidenediphenylene-co-lAcyolohexyldimethylcarbonate) 2 Poly(3,3'-ethylenedioxyphenylene thiocarbonate).

3. Poly(4,4-isoprcpylidenediphenylene carbonate-c0- terephthalate) 4. Poly(4,4-isopropylidenediphenylene carbonate).

5. Poly (4,4'-isopropylidenediphenylene thiocarbonate) 6. Poly(2,Z-butanebist-phenylene carbonate).

7 Poly(4,4'-isopropylidenediphenylene carbonatetlockethylene oxide).

8 Poly(4,4-isopropylidenediphenylene carbonate-blocktetramethyleneoxide) 9. Poly[4,4-isopropylidenebis(Z-methylphenylene)carbonate].

10.. Poly(4,4-isopropylidenediphenylene-co-l,4phenylene carbonate).

11 Poly(4,4-isopropylidenediphenylene-co-1,3-phenylene carbonate).

12 Poly(4,4'-isopropylidenediphenylene-co4,4-diphenylene carbonate).

13 Poly(4,4-isopropylidenediphenylene-c0-4,4-oxydipheny1- one carbonate).

14 Poly(4,4-isopropylidenediphenylene-co4,4-carbonyldiphenylene carbonate).

15 Poly(4,4-isopropylidenedipheuylene-cog i ethylenediphenylene carbonate).

l6 Poly[4,4-methylenebis(Z-methylphenylene)carbonate].

17 Poly[1,l-(p-bromophenylethane)bis(4phenylene) carbonate].

18 Poly[4,4-isopropylidenedip11enylene-co-sulfonylbis-(4- phenylene)earbonate].

Poly[1,t-cyelohexanebis(4-phenylene) carbonate]. P01y[4,4-isopropylidenebis(Z-chlorophenylene)carbonate]. Poly(hexafluoroisopropylidenedi-tphenylene carbonate). Poly(4,4-isopropylidenediphenylene-4,4-isopropylidene dibenzoate). Poly(4,4'-isopropylidonedibenzyl-4,4'-isopropylldene dibenzoate) Po1y[2,2-(3-methy1butane)bis+phenylene carbonate]. Poly[2,2-(3,3-dimethylbutane)bis-4-phenylene carbonate]. Poly]1,1-[1-(1-naphthy1)]bis-4-phenylene carbonate] Poly[2,2-(4-methylpentane)bis-4-phenylene carbonate]. Poly[4,4-(2-norbornylidene) diphenylene carbonate]. 29 Poly[4,4-(hexahydro-4,7-rnethanoindan-5-ylidene)- diphenylene carbonate].

Sensitized compositions formed according to the present invention can readily be used for enhancing the sensitivity and extending the spectral range of sensitivity of a variety of organic photoconductors and inorganic photoconductors including both nand p-type photoconductors. A typical example of an inorganic photoconductor would be zinc oxide. The present invention can be used in connection with many organic, including organometallic, photoconducting materials which having little or substantially no persistence of photoconductivity. Representative organo-metallic compounds are the organic derivatives of Group Illa, IVa, and Va metals such as those having at least one aminoaryl group attached to the metal atom. Exemplary organo-metallic compounds are the triphenylp-dialkylaminophenyl derivatives of silicon, germanium, tin and lead, the tri-p-dialkylaminophenyl derivatives of arsenic, antimony, phosphorus, bismuth boron, aluminum, gallium, thallium and indium. Useful photoconductors of this type are described in copending Goldman and Johnson U.S. pat. application Ser. No. 650,664, filed July 3, 1967 and Johnson application Ser. No. 755, now U.S. Pat. No. 3,607,257, filed Aug. 27, 1968.

An especially useful class of organic photoconductors is referred to herein as organic amine photoconductors. Such organic photoconductors have as a common structural feature at least one amino group. Useful organic photoconductors which can be spectrally sensitized in accordance with this invention include, therefore, arylamine, compounds comprising (1) diarylamines such as diphenylamine, dinaphthylamine, N,N-diphenylbenzidine, N- phenyl-l-naphthylamine, N-phenyl-Z-naphthylamine, N, 'N-diphenylp-phenylenediamine, 2-carboxy-5 chloro-'4'- methoxydiphenylamine, p-anilinophenol, N,N'-di-2-naphthyl-p-phenylenediamine, those described in Fox U.S. Pat.

9 3,240,597, issued March 15, 1966, and the like, and (2) triarylamines including (a) nonpolymeric triarylamines, such as triphenylamine, N,N,N,Ntetraphenyl-m-phenylenediamine, 4-acetyltriphenylamine, 4-hexanoyltriphenylamine, 4-lauroyltriphenylamine, 4-hexyltriphenylarnine, 4- dodecyltriphenylamine, 4,4-bis(diphenylamino)benzil, 4, 4-bis(diphenylamino)benzophenoneand the like, and (b) polymeric triarylamines such as poly[N,4"-(N,N, N'-triphenylbenzidine)], polyadipyltriphenylamine, polysebacyltriphenylamine, polydecamethylenetriphenylamine, poly-N-(4 vinylphenyl)diphenylamine, poly N (vinylphenyl)-a,u-dinaphthylamine and the like. Other useful amine-type photoconductors are disclosed in US. Pat. 3,180,730 issued Apr. 27, 1965.

Useful photoconductive substances capable of being sensitized in accordance with this invention are disclosed in Fox US. Pat. 3,265,496, issued Aug. 9, 1966, and include those represented by the following general formula:

R- N-T- Q,

wherein T represents a mononuclear or polynuclear divalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, binaphthyl, etc.), or a substituted divalent aromatic radical of these types wherein said substituent can comprise a member such as an acyl group naving from 1 to about 6 carbon atoms (e.g., acetyl, propiouyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, ethoxy, propoxy, pentogy, etc.), or a nitro group; M represents a mononuclear or polynuclear 'monovalent aromatic radical, either fused or linear (e.g., phenyl, naphthyl, biphenyl, etc.), or a substituted monovalent aromatic radical wherein said substituent can comprise a member, such as an acyl group having from 1 to about 6 carbon atoms (e.g., acetyl, propionyl, butyryl, etc.), an alkyl group having from 1 to about 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, etc.), an alkoxy group having from 1 to about 6 carbon atoms (e.g., methoxy, propoxy, pentoxy, etc.), or a nitro group; Q can represent a hydrogen atom, a halogen atom or an aromatic amino group such as MNH; [2 represents an integer from 1 to about 12; and R represents a hydrogen atom, a mononuclear or polynuclear aromatic radical, either fused or linear (e.g. phenyl, naphthyl, biphenyl, etc.), a substituted aromatic radical wherein said substituent comprises an alkyl group, an alkoxy group, an acyl group, or a nitro group, or a poly(4'-vinylphenyl) groupwhich is bonded to the nitrogen atom by a carbon atom of the phenyl group.

Polyarylalkane photoconductors are particularly useful in producing the present invention. Such photoconductors are described in US. Pat. 3,274,000 by Noe et al., issued Sept. 20, 1966, French Pat. 1,383,461 and in copending application of Seus and Goldman titled Photoconductive Elements Containing Organic Photoconductors, Ser. No. 627,857, now U.S. Pat. No. 3,542,544 filed Apr. 3, 1967. These photoconductors include leuco bases of diaryl or triaryl methane dye salts, 1,1,l-triarylalkanes wherein the alkaue moiety has at least two carbon atoms and tetraarylmethanes, there being substituted an amine group on at least one of the aryl groups attached to the alkane and methane moieties of the latter two classes of photoconductors which are non-leuco base materials.

Preferred polyarylalkane photoconductors can be represented by the formula:

wherein each of D, E and G is an aryl group and J is a hydrogen atom, an alkyl group, or an aryl group, at least one'of D, E and G containing an amino substituent. The aryl groups attached to the central carbon atom are preferably phenyl groups, although naphthyl groups can also be used. Such aryl groups can contain such substituents as alkyl and alkoxy typically having 1 to 8 carbon atoms, liydroxy, halogen, etc., in the ortho, meta or para positions, ortho-substituted phenyl being preferred. The aryl groups can also be joined together or cyclized to form a fluorene moiety, for example. The amino substituent can be represented by the formula:

wherein each L can be an alkyl group typically having 1 to 8 carbon atoms, a hydrogen atom, an aryl group, or together the necessary atoms to form a heterocyclic amino group typically having 5 to 6 atoms in the ring such as morpholino, pyridyl, pyrryl, etc. At least one of D, E and G is preferably p-dialkylaminophenyl group. When I is an alkyl group, such an alkyl group more generally has 1 to 7 carbon atoms.

Representative useful polyarylalkane photoconductors include the compounds listed in Table 3.

TABLE 3 Compound. number Name of compound 1 4,4J-benzylidene bis (N,N-diethyl-m-toluidine). 2 4,4 -diami1104-dimethylamino-2,2 -dimethyltriphenylmethane. 3 4,4"-bis(diethylamino)-2,6-dichloro-2,2"-dimethyltriphenylmethane. 4 4 ,4 -bis(diethylamiuo) -2 ,2 -dimetl1yldipheuylnaphthylmethane. 5 2,2-dimethyl-4,4,4-tris(dimethylamino)triphenylmethane. 6 4,4-bis(diethylamin0)4-dimethylamino-2,2-dimethyltriphenylmethane. 7 4',4"-bis(diethylamino)-2chloro-2,2"-dimethyl-4-dimethylaminotriphenylmethane. 8 4',4"-bis(diethylamino)-4-dimethylamino-2,2,2-trimethyltripheuylmethane. 9 4 ,4 -bis (dimethylamino) -2-ehloro-2 ,2 -dimetl1yltripheuylmethane. 10 4 ,4 -bis(dimethylamino) -2' ,2 -dimethyl-4-methoxytriphenylmethane. 11 Bis(4-diethylamino)-1,1,1-triphenylethane. l2 Bis(4-diethylamino)tetraphenylm ethane. 13 4,4-bis(benzylethylamino)-2,2-dimethyltriphenylmethane. 14 4.,4-bis(diethylamino)-2,2-diethoxytriphenylmethane. l5 4,4-his(dimethylamino)-1,l,1-triphenylethane.

l-(4-N,N-dimethylaminophenyl)-1,1-diphenylethane.

4-dimethylaminotetraphenylmethane. 18 4-diethylaminotetraphenylmethane.

Another class of photoconductors useful in this invention are the 4-diarylamino-substituted chalcones. Typical compounds of this type are low molecular weight nonpolymeric ketones having the general formula:

wherein R and R are each phenyl radicals including substituted phenyl radicals and particularly when R is a phenyl radical having the formula:

where R and R are each aryl radicals, aliphatic residues of l to 12 carbon atoms such as alkyl radicals preferably having 1 to 4 carbon atoms or hydrogen. Particularly advantageous results are obtained when R is a phenyl radical including substituted phenyl radicals and where R is diphenylaminophenyl, dimethylaminophenyl or phenyl.

Other photoconductors which can be used with the present aggregate compositions include rhodamine B, malachite green, crystal violet, phenosafranine, cadmium sulfide, cadmium selenide, parachloronil, benzil, trinitrofluorenone, tetranitrofluorenone, etc.

In preparing photoconductive comopsitions in accordance with this invention, useful results are obtained when the photoconductor is present in an amount equal to at least about /2% by weight of total solids added to the coating solvent. The upper limit of the amount of photoconductor present can be varied widely with up to 99% by weight of total solids being useful. A preferred weight range for the photoconductor is from about to about 80 weight percent. Of course, if it is desired to use the present aggregate compositions alone as the photoconductive substance, then no photoconductor would be added. In addition to the photoconductors described above, polymeric photoconductors, e.g., poly(N-vinylcarbazole), halogenated poly(N-vinylcarbazole), etc., can also be used if desired.

According to the process of this invention, a pyrylium dye as hereinbefore defined is dissolved in a suitable organic solvent, up to a predetermined concentration. The limit of concentration is deter-mined by the solubility of the dye in the dope resulting when the accompanying polymer is dissolved in the dye-containing solution. The amount of dye thus dissolved may be any amount from the solubility limit to about ten percent less than the solubility limit of the dye in the dope as above. It may be a greater or lesser amount than the amount which would dissolve in the solvent alone in the absence of polymer or other addendum, such as, for example, photoconductor, coating aid, and the like. Thus, the solubility limit of the dye in any dope will be dependent upon the particular materials present in the dope and their concentrations.

Solvents useful for preparing the dye-containing dope or compositions and elements to be coated therefrom in accordance with this invention can include a number of solvents such as aromatic hydrocarbons, e.g., benzene, toluene, including halogenated aromatic solvents such as chlorobenzene, dichlorobenzene, etc.; ketones such as dialkyl ketones having 1 to about 3 carbon atoms in the alkyl moiety, e.g., dimethyl ketone, methylethyl ketone, etc.; chlorinated hydrocarbons such as dichloroalkanes having 1 to about 3 carbon atoms, e.g., methylene chloride, ethylene chloride, trimethylene chloride, etc.; ethers, such as tetrahydrofuran, etc.; and mixtures of these and other solvents.

After the dye-containing solution has been prepared as described above, the hydrophobic polymer is dissolved in the solution. Mild stirring can be applied, if desired, to facilitate thorough mixing of the dissolved dye and polymer in the dope. Times of stirring can vary widely, with up to about 24 hours being employed if required. In general, the time required to dissolve the dye is somewhat longer than the time required to dissolve the polymer in the dye-containing solution. If necessary or desirable, a photoconductor can also be added at this stage, as can any other addendum which it is desired to incorporate.

The concentrated dope containing dye and dissolved polymer is next diluted to well below the solubility limit of the dye. The solvents used for diluting the dope are generally the same as are used in preparing the initial dye solution and polymer-containing dope produced therefrom, although it may be desirable in certain instances or even preferred to use difierent solvents. The amount of solvent added can vary widely depending upon the final solids content desired in the dope from which an electrophotographic element is to be prepared. In general, the amount added is between about 25 and about 100% by weight of the amount used in preparing the initial solution of dye. In addition, if necessary or desirable, further polymer or dye may be added to give the desired proportion of dye ot photoconductor in the coating dope, and at the same time to adjust the solids content.

In general, the total solids content of the resultant coating dope is about 10 to about 20% by weight of the dope. The total dye concentration is typically from about 0.5 to about 20% by weight of the total solids. The dye concentration of the final dope is preferably in the range in j \f/ ll l l I In this diagram, D represents the ratio of dye to solvent, with values increasing the upward direction. P represents the ratio of polymer to solvent, with values increasing to the right. In accordance with this invention, the dye is added to the solvent in an amount within about 10% of the solubility limit of the dye. This results in a ratio D of dye to solvent. The polymer and other ingredients are then added, resulting in a value of P, for example, P P or P which is not critical. Next, additional solvent is added to reduce the value of D from D to a final lower value D which is substantially below the solubility limit of the dye in the dope. In general, D is at least about 25% below the solubility limit of the dye in the dope. Depending on the starting value of P and the value desired in the final composition at point X, it may be necessary to add more polymer. Thus, the value of P at the first dilution stage can vary widely as shown by points 0, 1, 2 and 3. For example, the arrows starting from points 1 and 2 indicate addition of polymer is required, along with the solvent, in the second stage dilution to reach the desired polymer/ solvent ratio at X. The arrow from point 2 could likewise represent dilution with a polymercontaining dope having the concentration contained in the starting dope. This would reduce dye concentration only, leaving polymer concentration unaffected. The arrow from point 3 could result from the addition of solvent alone, as such addition would reduce the concentration of both dye and polymer in the solution. The final value of P after the second stage dilution will depend on that desired or required in the final composition.

The sensitizer-containing photoconductive dope thus prepared is next formed into an electrophotographic element by coating the dope onto a conductive support by.

known techniques. Suitable supporting materials for coating sensitizer-containing photoconductive layers in accordance with the method of this invention can include any of a wide variety of electrically conducting supports, for example, paper (at a relative humidity above 20% aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc; metal plates, such as aluminum, cop per, zinc, brass and galvanized plates; vapor deposited metal layers such as silver, nickel, aluminum and the like coated on paper or conventional photographic film bases such as cellulose acetate, polystyrene, etc. Such conducting materials as nickel can be vacuum deposited on transparent film supports in sufiiciently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side of such elements. An especially useful conducting support can be prepared by coating a support material such as poly(ethylene terephthalate) with a conducting layer containing a semiconductor dispersed in a resin or vacuum deposited on the support. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of maleic anhydride and a vinyl acetate polymer. Such kinds of conducting layers and methods for their optimum preparation and use are disclosed in US. 3,007,901 by Minsk, issued Nov. 7, 1961 and 3,262,807 by Sterman et al., issued July 26, 1966.

Coating thicknesses of the photoconductive composition on the support can vary widely. Normally, a coating in the range of about microns to about 300 microns before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about SOmicrons to about 150 microns before drying, although useful results can be obtained outside of this range. ,The resultant dry thickness of the coating is preferably between about 2 microns and about 50 microns, although useful results can be obtained with a'dry coating thickness between about 1 and about 200 microns.

For optimum formation of the feature material according to the method of this invention, drying conditions should be reasonably well controlled. For example, temperature and air flow are preferably adjusted so that not over about 80% of the solvent has been removed after about seconds after coating.

After the photoconductive elements prepared according to the method of this invention have been dried, they can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the xerographic process. In a process of this type, an electrophotographic element is held in the dark and given a blanket electrostatic charge by placing it under a corona discharge. This uniform charge is retained by the layer because of the substantial dark insulating property ofthe layer, i.e., the low conductivity of the layer in the dark. The electrostatic charge formed on the surface of the photoconductive layer is then selectively dissipated from the surface of the layer by imagewise exposure to light by means of a conventional exposure operation such as, for exaample, by a contact-printing technique, or by lens :projection of an image, and the like, to thereby form a latent electrostatic image in the photoconductive layer. Exposing the surface in this manner forms a pattern of electrostatic charge by virtue 'of the fact that light energy striking the photoconductor causes the electrostatic charge in the light struck areas to be con ducted away from the surface in proportion to the intensity of the illumination in a particular area.

The charge pattern produced by exposure is then developed or transferred to another surface and developed there, i.e., either the charged or uncharged areas rendered visible, by treatment with a medium comprising electrostatically responsive particles having optical density. The developing electrostatically responsive particles can be in the form of a dust, i.e., powder, or a pigment in a resinous carrier, i.e., toner. A preferred method of applying such toner to a latent electrostatic image for solid area development is by the use of a magnetic brush. Methods of forming and using a magnetic brush toner applicator are described in the following U.S. patents: 'Young 2,786,439, issued Nov. 18, 1952; Giaimo 2,786,440, issued Mar. 26, 1957; Young 2,786,441, issued Mar. 26, 1957; and Greig 2,874,063, issued Feb. 17, 1959. Liquid development of the latent electrostatic image may also be used. In liquid development, the developing particles are carried to the image-bearing surface in an electrically insulating liquid carrier. Methods of development of this type are widely known and have been described in the patent literature, for example, U.S. Pat. 2,907,674 by Metcalfe et al., issued Oct. 6, 1959 and in Australian Pat. 212,315. In dry developing processes, the most widely used method of obtaining a permanent record is achieved by selecting a developing particle which has as one of its components a low melting resin. Heating the powder image then causes the resin to melt or fuse into or onto the element. The powder is, therefore, caused to adhere permanently to the surface of the photoconductive layer. In other cases, a transfer of the electrostatic charge image formed on the photoconductive layer can be made to a second support such as paper which would then become the final print 14 after development and fusing. Techniques of the type indicated are well known in the art.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A 0.3 gram portion of the dye 4-(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium fluoroborate is dissolved with stirring in a solvent mixture comprising 24.0 grams of dichloromethane and 16.0 grams of 1,1,2-trichloroethane. After addition of the dye is complete, the solution is stirred for 16 hours to ensure complete dissolution. This concentration of dye is about 95% of the solubility limit of the dye in the dope resulting when the resin and photoconductor are added in the next step. After the above period of stirring, 6.0 grams of poly(4,4'-isopropylidenediphenylene carbonate) (Lexan 145, General Electric Co.) and 4.0 grams of 4,4-diethylamino-2,2-dimethyltriphenylmethane) are added, and the resulting dope stirred for another hour. The concentration ratio of sensitizer to solvent is calculated to be 0.0075, and the concentration ratio of resin to solvent is 0.15. The dope is allowed to stand without further agitation for 16' hours at a temperature of about 23 C. The dope is diluted to working strength by addition of a further 10.0 grams of dichloromethane and 6.7 grams of 1,1,2-trichloroethane. At this stage, the ratio of sensitizer to solvent is reduced to 0.0053, or 71% of the solubility limit of the dye in the dope, and the ratio of resin to solvent is reduced to 0.106. The percent solids, originally 20.5%, is reduced to 15.4% by the addition of this amount of solvent. The composition is coated at a wet thickness of 75 microns on a conductive support comprising poly(ethylene terephthalate) bearing a layer of nickel coated by evaporation in vacuum to an optical density of about 0.4. After drying, the resultant electrophotographic element is then electrostatically charged under a positive corona source until the surface potential, as measured by an electrometer probe, reaches about 600 volts. The charged element is then imagewise exposed to a pattern of light and shadow to produce an electrostatic charge pattern thereon corresponding to the light pattern. The charge pattern is rendered visible by contacting the charged surface with a developer comprising electroscopic marking particles having optical density. A good reproduction of the light pattern results. A second coating composition is prepared in the identical manner, except that the entire quantity of solvent used to prepare the above composition is present when the dye is initially dissolved. The composition is coated as above to prepare a control element. The control, when charged, imagewise exposed and developed as above, also gives a good reproduction. When inspected visually, the two elements appear similar in color to the unaided eye, but the element prepared by the method of this invention appears to have a glossy finish, while the control element appears to have a matte finish. This smooth sur face is an advantage. For example, during cleaning to prepare the element for reuse, residual toner is more readily removed from the surface. Cross-section photomicrographs are also made of each element. Visual inspection of these photomicrographs indicate that the average size of the aggregate formed according to the method of this invention is approximately 0.2 micron, while that of the control element is a micron or larger. Clearly, the method of this invention results in aggregate species having greatly reduced particle size.

EXAMPLE 2 The procedure of Example 1 is repeated, using the same initial quantities of solvents, and dissolving therein 0.51 gram of the dye 2-(4-ethoxypheny1)-4-(dimethylaminophenyl)-6-phenylthiapyrylium perchlorate. This concentration of the dye is about of the solubility limit of the dye in the dope resulting when the resin and photoconductor are added in the next step. After stirring, 10.30 grams of the resin and 6.70 grams of the photoconductor of Example 1 are added in the manner described therein. The ratio of sensitizer to solvent is 0.0127, and the ratio of resin to solvent is 0.258. The dope is allowed to stand as in Example 1 and then diluted by the addition of 35.50 grams of dichloromethane and 23.50 grams of 1,1,2-trichloroethane. The ratio of sensitizer to solvent is reduced to 0.0053, or 58% of the solubility limit of the dye in the dope and the ratio of resin to solvent is reduced to 0.106. The percent solids, originally 22.1%, is reduced to 15.4% by the addition of solvent. An element is prepared as in Example 1 using the dope prepared herein, and a similar element is prepared using the same quantities of ingredients but without the two-stage dilution technique of the invention. Each element is charged as in Example 1 and exposed to a 3000 K. tungsten light source through a stepped density gray scale. The exposure causes reduction of the surface potential of the element under each step of the gray scale from its initial potential, V0, to some lower potential, V, whose exact value depends on the actual amount of exposure in meter-candleseconds received by the area. The results of these measurements are then plotted on a graph of surface potential V vs. log exposure for each step. The relative speed of the photoconductive composition can then be expresed in terms of the reciprocal of the exposure required to reduce the surface potential to any fixed arbitrarily selected value. Herein, the relative speed is a function of the reciprocal of the exposure in meter-candle-seconds required to reduce the "600 volt charged surface potential by 100 volts. The relative positive speeds of the element prepared according to the method of the invention and of the control are 500 and 7, respectively, while the corresponding relative negative speeds are 200 and 5.5, respectively. The spectral absorption peak of the element prepared according to the method of this invention is 685 nm. Cross-section photomicrographs at a magnification of 25 clearly show the presence of submicron-sized aggregates in the element prepared according to the invention, while no such small aggregates are observed in the control element.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be eifected within the spirit and scope of the invention.

We claim:

1. A method of preparing from a coating dope a heterogeneous organic photoconductive composition containing a co-crystalline complex of dye and polymer, said dope comprising at least one pyrylium dye and an electrically insulating, film-forming, organic polymer containing an alkylidene diarylene moiety, the method comprising the steps of:

(a) dissolving the dye in an organic solvent at a dye to solvent ratio substantially equal to the solubility limit of the dye in the dope;

(b) adding polymer to the dye solution thereby solubilizing the polymer with the dye;

(c) thoroughly mixing the combination of dye and polymer;

(d) adding more solvent to the above combination to reduce the dye to solvent ratio to a value less than about 75% of the solibility limit of the dye in the dope and thereby forming a composition having a total solids content of about to about 20% by weight of the dope with a dye concentration of about 1 to about 10% by weight of the total solids; and

(e) preparing a thin film about 10 to about 300 microns thick of the dope -whereby a heterogeneous photoconductive composition. is formed which has a continuous phase of polymer and a particulate discontinuous phase containing a co-crystalline complex of dye and polymer.

2. The method as described in claim 1 wherein said dye is selected from the group consisting of a thiapyrylium dye salt, a selenapyrylium dye salt and a pyrylium dye salt.

3. The method as described in claim 1 wherein an organic photoconductor is added prior to coating said dope.

4. The method as described in claim 1 wherein said discontinuous phase consists of particles of the co-crystalline complex predominantly in the size range of about 0.01 to about 0.75 micron.

5. A method of preparing from a coating dope a heterogeneous photoconductive composition comprising a photoconductor and a co-crystalline complex of a pyrylium dye and a polymer containing the following moiety in the recurring unit:

Ra R4 R1 1'1.

wherein:

each of R and R when taken separately, is selected from the group consisting of a hydrogen atom, an alkyl radical of from 1 to 10 carbon atoms and a phenyl radical and R and R when taken together, are the carbon atoms necessary to form a cyclic hydrocarbon radical, the total number of carbon atoms in R and R being up to 19;

R and R are each selected from the group consisting of hydrogen, alkyl radicals of from 1 to 5 carbon atoms, alkoxy radicals of from 1 to 5 carbon atoms and a halogen; and

R is selected from the group consisting of divalent radicals having the formulae:

and

the method comprising the steps of:

(a) dissolving said dye in an organic solvent at a dye solvent ratio of at least about of the solubility limit of the dye in the dope;

(b) adding a photoconductor and said polymer to the dye solution;

(0) thoroughly mixing the combination of dye, polymer and photoconductor, thereby solubilizing the polymer with the dye;

(d) adding additional solvent to the above combination to reduce the dye to solvent ratio to a value less than about 75% of the solubility limit of the dye in the dope and thereby forming a composition having a total solids content of about 10 to about 20% by weight of the dope with a dye concentration of about 1 to about 10% by weight of the total solids; and

(e) coating a thin film about 10 to about 300 microns thick of the resultant dope whereby a heterogeneous photoconductive composition is formed which has a continuous phase of said polymer and a particulate discontinuous phase containing a co-crystalline complex of dye and polymer, said discontinuous phase having a particle size of less than about 1 micron.

6. The method as described in claim wherein said dye has the formula:

wherein:

R and R are aryl radicals selected from the group consisting of phenyl and substituted phenyl having at least one substituent selected from the group consisting of an alkyl radical of from 1 to 6 carbon atoms and an alkoxy radical of from 1 to 6 carbon atoms;

R is an alkylamino-substituted phenyl radical having from 1 to 6 carbon atoms in the alkyl moiety;

X is selected from the group consisting of sulfur and oxygen; and

Z- is an anion.

7. The method as described in claim 5 wherein said polymer is a carbonate polymer and said photoconductor is an organic photoconductor.

8. The method as described in claim 5 wherein said dye is selected from the group consisting of perchlorate, fluoroborate and p-toluenesulfonate salts of an anion selected from the group consisting of 4-[4-bis(2-chloroethyl)aminophenyl]2,6-diphenylthiapyrylium, 4-(4-dimetyhlaminophenyl)-2,6-diphenylthiapyrylium, 2,6-bis(4-ethylphenyl)-4-(4-dimethylaminophenyl) thiapyrylium,

18 4-(4-dirnethylaminophenyl)-2-(4-ethoxyphenyl)-6- phenylthiapyrylium, 4-(4-dimethylamino-Z-methylphenyl)-2,6-diphenylpyrylium,

4- [4sdi 2-chloroethy1) aminophenyl] -2- 4-methoxyphenyl)-6-phenylthiapyry1ium,

4(4-dimethylaminophenyl)-2,6-diphenylthiapyrylium,

4-(4-dimethylaminophenyl)-2,6-diphenylpyrylium,

2- (2,4-dimethoxyphenyl) -4- (4-dimethylaminophenyl) benzo(b)pyrylium, and

4- (4-dimethylaminophenyl) -2- 4-methoxyphenyl) -6- phenylthiapyrylium.

9. The method as described in claim 5 wherein the solvent is selected from the group consisting of dialkyl ketones, aromatic hydrocarbon solvents, chlorinated hydrocarbon solvents and ethers.

10. The method as described in claim 5 wherein the total solids content of the resultant dope is about 10 to about 20% by weight of the dope.

References Cited UNITED STATES PATENTS 3,052,540 9/1962 Greig 96--1.7 3,125,447 3/ 1964 Stewart 961.7 3,250,615 5/1966 Van Allan et al. 96--l.7 3,488,705 1/1970 Fox et a1. 961.6 3,503,740 3/1970 De Selms 96l.6 X

GEORGE F. LESMES, Primary Examiner M. WITTENBERG, Assistant Examiner US. Cl. X.R.

252-501; 260-3451, 345.9, 327 TH, 34.2, 37 PC 

